Meanwhile, whisk the remaining 1/2 cup milk and cornstarch in a small bowl. Add to the pan and cook, stirring vigorously, until bubbling and thickened, 1 to 2 minutes. Reduce heat to low, add cheese and cook, stirring, until melted. Stir in drained tomatoes, lime juice, salt, chili powder and cayenne (if using). Serve warm, garnished with scallions and cilantro.